Olefinic compounds (alkenes) are widely used in a number of chemical industries. To name a few, for the production of petrochemical products, such as synthetic rubbers, plastics, motor fuel blending additives. Among the olefins, propylene (propene) is the world's second largest petrochemical commodity, being the precursor of polypropylene, which is used in such everyday products as packaging materials and outdoor clothing.
Nowadays, light olefins (e.g. ethylene, propylene, isobutene) are commonly obtained by steam cracking (SC) and fluid catalytic cracking (FCC) of light oil fractions. For example, most propylene is produced as co-product in steam crackers (>55%) and as by-product in FCC units (˜35%), while only small fraction (<10%) is produced by alternative technologies, such as propane dehydrogenation.
In the case of both SC and FCC, coking and side reactions are among major drawbacks. Another disadvantage of steam cracking is its relatively low selectivity for the desired products. A wide range of products is produced with limited flexibility. This is inherent to the non-catalytic nature of the process.
Catalytic dehydrogenation of alkanes is becoming a growing branch in petrochemical industry as a route to obtain alkenes from low-cost feedstocks of saturated hydrocarbons (alkanes), according to the reaction equation (1):CnH2n+2CnH2n+H2  (1)
As compared to conventional cracking technologies, catalytic dehydrogenation may provide better selectivity at lower temperatures, lowering also the coke deposition rate.
Specific features of dehydrogenation reactions determine the reaction conditions, process design and the nature of catalysts. Paraffins dehydrogenation reactions are highly endothermic (about 30 kcal/mol for propane) and the yields of target products are limited by thermodynamic equilibrium. According to Le Chatelier's principle, higher conversion will require either higher temperature or lower pressures. In a somewhat abbreviated form for the production of mono-olefins, this can be expressed as follows (2):
                              x          e          2                =                              K            p                                              K              p                        +            P                                              (        2        )            
wherein xe is the equilibrium conversion, P the total absolute pressure and Kp is the equilibrium constant for the dehydrogenation reaction.
An equilibrium diagram for the generic reaction (1) at atmospheric pressure is shown in FIG. 1. The temperature necessary to obtain an iso-conversion is dependent on the number of carbon atoms in the molecule: the lower is this value, the higher is the temperature. Temperatures as high as 900° C. and 750° C. are required to achieve a conversion of 90% for ethane and propane at equilibrium, respectively (at atmospheric pressure). To obtain 70% conversion, the corresponding temperatures are 790 and 660° C.
These conditions might be critical for the paraffins and olefins stability: favored by high temperatures, several side reactions take place. E.g., oligomerization to heavier compounds, cracking to lighter hydrocarbons, skeletal isomerization, aromatization, alkylation of the formed aromatic rings, eventually leading to coke formation, and these may lower the yields to olefins. Accordingly, the use of a specific catalyst is necessary in order to keep a suitable conversion while obtaining high selectivity towards the desired olefin. Furthermore, the unavoidable formation of coke on the catalyst surface results in progressive reduction of catalytic activity. Thus, a periodical regeneration of the catalyst is required, generally realized by oxidation of the deposited carbonaceous compounds. In view of the temperatures to be applied in the conversion of propane into propene, the foregoing problem is particularly incurred when producing propene.
Taking two major olefins as illustrative examples, there are two leading processes industrially developed for dehydrogenation of propane to propylene and isobutane to isobutene and they are both operated on a large scale. E.g. the ABB Lummus Catofin process is a cyclic process that uses Cr2O3/Al2O3 catalyst (activated alumina impregnated with 18-20 wt % chromium) in, at least, three fixed bed reactors operating under slight vacuum. With these three reactors, one reactor is processing the alkane feed, one has its catalyst regenerated in situ with air, and the third is purged, so as to give a continuous plant throughput. Fresh and recycle feed are preheated and fed to the reactor at 0.35-0.7 bar pressure. During reaction, coke deposited on the catalyst and combustion of the coke during regeneration re-heats the catalyst bed. During the hydrocarbon processing step, fresh feed and recycle feed from a C3 splitter are vaporized by exchange with various process streams and then raised to reaction temperature in the charge heater. The reactor effluent is cooled, compressed and sent to the purification and separation section.
The UOP Oleflex continuous process uses a Pt/Sn/Al2O3 catalyst in 3-4 adiabatic (though close to isothermal) moving bed reactors with feed preheat, inter stage heating and continuous catalyst regeneration. The process gets close to thermodynamic equilibrium. Fresh feed is mixed with recycled hydrogen (to reduce coking) and unconverted feed at slightly positive pressure. This process is characterized by high capital outlays because of sophisticated apparatus. Moreover, this technology requires a high mechanical strength of the catalyst.
Typically, to suppress coking during dehydrogenation, hydrogen or water vapor is added to the reaction mixture. The amount of these additives should be optimized. In the case of hydrogen, this is dictated by the fact that it is a reaction product, and, therefore, when added, it shifts the reaction equilibrium to the initial alkane, thereby decreasing the degree of conversion. The introduction of water vapor decreases the selectivity in the target product because of the formation of carbon oxides.
However, it must be taken into account that an additional mechanism of deactivation may occur, due to catalyst particle agglomeration (sintering). This may lead to a reduction in active specific surface area and, as a result, catalyst activity. The resulting deactivated catalytic pellets cannot be regenerated just by combustion of hydrocarbons over the deactivated catalytic bed (like in case of coking); this type of deactivation is rather irreversible. Moreover, periodic regenerations of the catalytic bed (to burn off deposited coke) make the sintering deactivation even more severe.
Updating the existing dehydrogenation methods aimed at increasing the yield of olefin hydrocarbons, process selectivity and reducing the amount of coke deposited on the catalyst is a very important problem today.
One of the various approaches to overcome the limitations of alkanes dehydrogenation is represented by the oxidative dehydrogenation (ODH) of alkanes. With the introduction of an oxidant into the reaction mixture, the reaction becomes exothermic and is able to proceed at much lower temperatures. This in turn reduces the side reactions, such as cracking of alkanes and coke formation. Moreover, the thermodynamic limitations of dehydrogenation can be overcome, since removal of hydrogen from the reactive mixture (by oxidizing H2 to water) shifts the equilibrium toward formation of products (alkenes). Several compounds may be used as oxidizing agents: molecular oxygen, halogens, sulfur compounds; the preferred reactant for industrial purposes is usually considered to be molecular oxygen, because of its low cost and little environmental impact.
U.S. Pat. No. 3,904,703 discloses a method for conducting a dehydrogenation reaction involving the contact of a hydrocarbon feedstock with a sequence of a dehydrogenation catalyst in a first zone, then in a second zone with an oxidation or reducible catalyst and then in a third zone with an adsorbent. The sequence may be repeated as many times as desired depending on the space in the reactor or reactors. However, oxidative dehydrogenation has drawbacks of its own, such as a difficulty of controlling the consecutive oxidation of alkanes/alkenes to carbon oxides, the removal of reaction heat, flammability of the reaction mixture, and the possibility of reaction runaway.
Another approach to overcome the limitations of the dehydrogenation of alkanes is represented by the use of membrane reactors, in which the chemical reaction is coupled with the separation of one of the end products, such as hydrogen. In this manner, it is possible to shift the reaction—in the above equation (1)—to the right side and consequently the conversion rate or final product yield may be enhanced. Significant advantages deriving from the use of membrane reactors are the following: (i) conversion enhancement of equilibrium limited reactions; (ii) achievement of the same performance attained in a traditional reactor at milder operating conditions, such as lower temperature; (iii) reduced capital costs due to the combination of reaction and separation in only one system. Use of Pd or Pd-alloy membranes in catalytic membrane reactors, where the membrane “extracts” hydrogen from a reaction, has been proven experimentally and theoretically to be efficient in enhancing conversions and/or lowering operating temperatures of several types of endothermic, equilibrium-limited reactions of petrochemical industry.
In the scientific and patent literature a great many disclosures deal with hydrocarbon dehydrogenation carried out in the presence of a membrane for hydrogen separation.
GB 1,199,683 discloses a process for catalytic dehydrogenation, dehydrocyclization or hydrodealkylation of hydrocarbons having from 2 to 20 carbon atoms, in which the same catalyst acts as membrane for hydrogen separation being based on metals permselective to hydrogen. In one example, a constant catalyst activity for 200 hours was disclosed.
U.S. Pat. No. 5,202,517 discloses a process for dehydrogenation of an alkane or a mixture of alkanes using a tubular ceramic membrane impregnated with a catalytically active metallic substance and a pelletized catalyst material adjacent to the side of the membrane. In the specific case of ethane dehydrogenation to ethylene, conversion levels equivalent to two to five times normal thermodynamic equilibrium in the combined exit gas at temperatures of 500 to 600° C. were obtained.
GB 2 201 159 discloses a process and apparatus for the dehydrogenation of organic compounds using a ceramic membrane permselective to hydrogen. The use of a ceramic membrane is justified as a heat supplier for a dehydrogenation reactor when conducted through by an electric current. In one example, an about 46% increase in propane conversion is observed.
U.S. Pat. No. 5,430,218 discloses a catalytic paraffins dehydrogenation process characterized by hydrocarbon improvement conversion under the hydrogen removal by a thermally stable polymer-porous solid membrane. An increase in feedstock conversion is observed in the presence of the membrane, which furthermore does not adversely affect the overall selectivity.
US 2002/0099248 A1 discloses an integrated process for olefin and polyolefin production via polymerization steps. The process utilizes membrane type permeators downstream of the polymerization reactor for separation of the unconverted olefins from paraffins. In one embodiment the produced propylene obtained after the hydrogen removal is employed for the production of acrolein and acrylic acid.
GB 1,039,381 refers to the use of a membrane reactor in a variety of process that produce hydrogen. As an example, hydrocarbon dehydrogenation reactions are mentioned. It is referred to, inter alia, that lower temperatures can be realized, which would allow the use of a wider choice of catalysts. The process is conducted using a single unit, viz. a compartmented reactor, containing a reaction chamber and a diffusion chamber, or containing a plurality of reaction zones.
U.S. Pat. No. 5,430,218 refers to the use of polymeric membranes for hydrogen separation. As a result, a decoupling of the dehydrogenation reaction processes and membrane separation processes is required. This is presented as an advantage, in view of the necessary regeneration of the dehydrogenation catalyst as a result of coke formation. Hence, the reference essentially accepts the phenomen of coke formation as is, and does not teach the skilled person how to reduce coke formation.
Despite all that is known in the area of membrane reactors, including its use in catalytic dehydrogenation of alkanes, the main lack in the traditional technology is still the higher amount of coke deposited on the catalyst. Reducing this amount would provide for the realization of a continuous process avoiding the necessity of catalyst regeneration.
Furthermore, it is desired to provide processes for the catalytic dehydrogenation of alkanes that allow a higher alkane conversion, yet without a similar promotion of coke formation. Also, it would be desired to provide a method to modernize existing olefin production plants in terms of lower coke formation and/or higher conversion rates.
It is particularly desired to provide a process that allows the foregeing drawbacks to be obviated specifically in the production of propene from propane.